Working on a 305 with a Holley 600cfm vac sec. Ok I set the base timing to zero with the vacuum line off the distributor. How much advance should I have with the vaccum line hooked up to the distributor? The vaccum guage sits around 13/14 fluctuates a little is that normal? I would think a little is normal in a rebuilt motor. I could be wrong though. Should I tune the car with the exhaust off the car. Should I check my valve lash again would that affect the vacuum? Yes I did check for a vac leak with some carb cleaner I didnt find one. Could I get some tuning help here?

Thanks in advance

 

Rad,

Do you run a stock cam? If so, that vacuum at idle seems a bit low. A cam with less LSA or more duration (and overlap) will normally produce a lower idle vacuum. Are you using a hydraulic lifter or mechanicals? The slight fluctuation is normal with an aftermarket cam and a sensitive gauge,but could also be a sign of a tight lifter or sticky or poorly sealing valve.

It wouldn't hurt to check the valve lash, and adjusting hydraulic lifter lash while running hot is the most accurate method (but the most mess). If you have an aftermarket hydraulic cam, keep the lash preload (turns after zero lash) down to no more than 1/4-1/2 turn. This will preload the lifters enough to take up lash, but prevent pump-up and valve unseating at higher RPM. If you're running a mechanical lifter cam,make sure the engine is HOT when adjusting lash,and lean toward the loose end of the tolerance when adjusting for the first run - about 0.008" for intakes and 0.018" for exhausts.

As for the vacuum advance, if you connected the distributor advance servo to a timed vacuum port on the Holley, you won't get any vacuum advance until the throttle are opened slightly. Test the vacuum at the port with your gauge to see the phenomenon.

And it would probably be best to tune with everything connected as it will be once the car is completed. Running with no exhaust might skew the results.

 

I think the problem might be that I do not have any advance on the timing initial that is I was gonna try to put a vacuum line on the manifold and adjust my timing for optimal vacuum and check the timing after I adjust the idle. Have you ever done this before? I am running a Summit cam with hydraulic lifters. I think if I put some advance timing in it will smooth out my idle and fix the vacuum problem. The cam is not stock it has a lift of 442 intake and 460 exhaust with lsa of 110 or 112 I cant remember of hand. slightly bigger than stock. Right now it really hard to here any valve train issues. The exhaust is quite loud. I cant see how anyone can drive open headers. Thanks for your input and have a good day Jeff.

Ed

 

Rad,

Yes, I've set timing by peak vacuum, and usually discovered that it was a bit too advanced as a result. I can't say exactly how much, since all engines are a bit different. You could at least get close that way, with the understanding that peak idle vacuum may be at a point too advaced for WOT operation without detonation. SBC engine do seem to like a lot of advance at idle.

Yes, you should at least have some chambers or fiberglass-filled tubes behind the collectors so you have a chance of hearing anything unusual.

Again, with hydraulic lifters, a tight lash can cause a lower vacuum and pulsation of fluctuation. It may even contribute to a choppy or rough idle. I can understand how setting the lash hot and running would be difficult at this point, however.

 

Wow you and I must be about the same age. lol I listened to him during High School many moons ago. I put the vacuum guage where the pcv hooks into the carb much better vac readings here some flucuation but within a pound. I got around 14" with advancing the distributor. I could have achieved more vacuum If I wanted to die of smoke inhalation lol. I think I might need a new distributor though. I can hear the weights rattleing inside. I will tune some more once I have the exhaust on though. This could be the cause of low vac too. Do you think? This engine likes too rev high though. I think once I adjust the valve lifters, I maybe able to get more vacuum out of it. I dont think I will achieve the ideal vacuum with this cam. It maybe a little to big for a 305. I contributed my erratic vacuum gauge to not plugging the vac advance port on the carb earlier. I am learning though. Thanks for all your help. All in all the holley doesnt seem to complicated, its everything else you have to adjust and re-adjust trial an error I just dont wanna blow this thing up. I did notice the engine ran a lot cooler with some advance too. I guess another characteristic of a Chevy hah. Thanks have a great day.

Ed

 

Holleys are quite simple. Sometimes too simple. The transition between idle and main circuits is often not that smooth due to the limitations on metering adjustments. I'm partial to the old Carter Thermoquads, with a metering rod for just about every aspect of operation.

A fresh engine will often be a little warm, and retarded timing doesn't help either.

Yes, I saw the deputy for the first time in concert in 1973 at a local armory - long before most of these civic centers and entertainment boxes existed. Yeah, we're gettin' old...

 

I did notice the transition too not the smoothest by any means. What do you thing of the little rattle coming from the distributor though? I do get advance with the vacuum blocked. Something I should worry about or not?

 

You could check the weights and springs. The pins on which the weights pivot are hardened,but they can wear. Usually, the weights wear more, and you might be able to take up some of the slop with a new pair of advance weights. Most speed shops used to have sets of these readily available to alter the advance curve of the distributor. If you mic the pins and they are still a nominal size, new weights might be the trick.

 

Well, I have a new weight and spring kit on the distributor but I am not real happy with the quality. They were sloppy when I put them on but I think it was the quality of the parts and not the pins. The bushings were sloppy and made of cheap plastic. I guess I know to stay away from the spectre brand. Think I will pickup a Moroso kit. Any suggestions as to which springs to start out with?

 

The last set I bought had three sets of springs included with two weights and bronze bushings. There were green, red, and gold colored springs. The weights themselves determine the total advance, but the springs determine the RPM at which total advance is achieved. You can mix the sets of springs to get various RPM combinations for total advance RPM. IIRC, the maximum range was full advance (37°) @ 4,400 RPM (2,200 distributor RPM). There should be instructions included with the kit to give approximate engine RPM for full advance with each spring combination. Looking at some old note, I set up an old 355 for 45° total advance (8° BTDC base setting) at 4,100 RPM with good results, but you'll have to experiment with your combination.